Abstract

According to L-edge sum rules, the number of 3d vacancies at a transition metal site is directly proportional to the integrated intensity of the L-edge X-ray absorption spectrum (XAS) for the corresponding metal complex. In this study, the numbers of 3d holes are characterized quantitatively or semi-quantitatively for a series of manganese (Mn) and nickel (Ni) complexes, including the electron configurations 3d10 3d0. In addition, extremely dilute (<0.1% wt./wt.) Ni enzymes were examined with two different approaches: 1) by using a high resolution superconducting tunnel junction (STJ) X-ray detector to obtain XAS spectra with very high signal-to-noise ratio, especially in the non-variant edge jump region; and 2) by adding an inert tracer to the sample that provides a prominent spectral feature to replace the weak edge jump for intensity normalization. In this publication, we present for the first time: 1) L-edge sum rule analysis for a series of Mn and Ni complexes that include electron configurations from an open shell 3d0 to a closed shell 3d10; 2) a systematic analysis on the uncertainties, especially on that from the edge jump, which was missing in all previous reports; 3) a clearly-resolved edge jump between the pre-L3 and the post-L2 regions from an extremely dilute sample; 4) an evaluation of an alternative normalization standard for L-edge sum rule analysis. XAS from two copper (Cu) proteins measured with a conventional semiconductor X-ray detector are also repeated as bridges between the Ni complexes and the dilute Ni enzymes. The differences between measuring 1% Cu enzymes and measuring < 0.1% Ni enzymes are compared and discussed. This study extends L-edge sum rule analysis to virtually any 3d metal complex and any dilute biological samples that contain 3d metals.

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